P. Vidhya Sri* et al. (IJITR) INTERNATIONAL JOURNAL OF ...

P. Vidhya Sri* et al. (IJITR) INTERNATIONAL JOURNAL OF INNOVATIVE TECHNOLOGY AND RESEARCH

Volume No.4, Issue No.6, October – November 2016, 4889-4894.

2320 –5547 @ 2013-2016 http://www.ijitr.com All rights Reserved. Page | 4889

A Seven-Level Inverter For Photo Voltaic System P.VIDHYA SRI

PG Scholar, Department of EEE

G.K.C.E, Sullurpet, Andhrapradesh, INDIA.

K. SWAPNA

Assistant Professor, Department of EEE

G.K.C.E, Sullurpet, Andhrapradesh, INDIA.

Abstract: This paper proposes a new solar power generation system, which is composed of a dc/dc power

converter and a new seven-level inverter. The dc/dc power converter integrates a dc–dc boost converter

and a transformer to convert the output voltage of the solar cell array into two independent voltage

sources with multiple relationships. This new seven-level inverter is configured using a capacitor selection

circuit and a full-bridge power converter, connected in cascade. The capacitor selection circuit converts

the two Output voltage sources of dc–dc power converter into a three-level dc voltage, and the full-bridge

power converter further converts this three-level dc voltage into a seven-level ac voltage. In this way, the

proposed solar power generation system generates a sinusoidal output current that is in phase with the

utility voltage and is fed into the utility. The salient features of the proposed seven-level inverter are that

only six power electronic switches are used, and only one power electronic switch is switched at high

frequency at any time. A prototype is developed and tested to verify the performance of this proposed

solar power generation system.

I. INTRODUCTION

The extensive use of fossil fuels has resulted in the

global problem of greenhouse emissions.

Moreover, as the supplies of fossil fuels are

depleted in the future, they will become

increasingly expensive. Thus, solar energy is

becoming more important since it produces less

pollution and the cost of fossil fuel energy is rising,

while the cost of solar arrays is decreasing. In

particular, small-capacity distributed power

generation systems using solar energy may be

widely used in residential applications in the near

future.

The power conversion interface is important to grid

connected solar power generation systems because

it converts the dc power generated by a solar cell

array into ac power and feeds this ac power into the

utility grid. An inverter is necessary in the power

conversion interface to convert the dc power to ac

power. Since the output voltage of a solar cell array

is low, a dc–dc power converter is used in a small-

capacity solar power generation system to boost the

output voltage, so it can match the dc bus voltage

of the inverter. The power conversion efficiency of

the power conversion interface is important to

insure that there is no waste of the energy

generated by the solar cell array. The active devices

and passive devices in the inverter produce a power

loss. The power losses due to active devices

include both conduction losses and switching

losses. Conduction loss results from the use of

active devices, while the switching loss is

proportional to the voltage and the current changes

for each switching and switching frequency. A

filter inductor is used to process the switching

harmonics of an inverter, so the power loss is

proportional to the amount of switching harmonics.

The voltage change in each switching operation for

a multilevel inverter is reduced in order to improve

its power conversion efficiency and the switching

stress of the active devices. The amount of

switching harmonics is also attenuated, so the

power loss caused by the filter inductor is also

reduced. Therefore, multilevel inverter technology

has been the subject of much research over the past

few years. In theory, multilevel inverters should be

designed with higher voltage levels in order to

improve the conversion efficiency and to reduce

harmonic content and electromagnetic interference

(EMI).Conventional multilevel inverter topologies

include the diode clamped the flying-capacitor, and

the cascade H-bridge types. Diode-clamped and

flying capacitor multilevel inverters use capacitors

to develop several voltage levels. But it isdifficult

to regulate the voltage of these capacitors. Since it

is difficult to create an asymmetric voltage

technology in both the diode-clamped and the

flying capacitor topologies, the power circuit is

complicated by the increase in the voltage levels

that is necessary for a multilevel inverter. For a

single-phase seven-level inverter, 12 power

electronic switches are required in both the diode-

clamped and the flying-capacitor topologies.

Asymmetric voltage technology is used in the

cascade H-bridge multilevel inverter to allow more

levels of output voltage, so the cascade H-bridge

multilevel inverter is suitable for applications with

increased voltage levels. Two H-bridge inverters

with a dc bus voltage of multiple relationships can

be connected in cascade to produce a single phase

seven-level inverter and eight power electronic

switches are used. More recently, various novel

topologies for seven level inverters have been

proposed. For example, a single-phase seven-level

grid-connected inverter has been developed for a

photovoltaic system. This seven-level grid-

connected inverter contains six power electronic

switches. However, three dc capacitors are used to

construct the three voltage levels, which results in

that balancing the voltages of the capacitors is

more complex. In a seven-level inverter topology,




configured by a level generation part and a polarity

generation.

II. THE PHOTOVOLTAIC ARRAY

A PV array consists of a number of PV modules,

mounted in the same plane and electrically

connected to give the required electrical output for

the application. The PV array can be of any size

from a few hundred watts to hundreds of kilowatts,

although the larger systems are often divided into

several electrically independent sub arrays each

feeding into their own power conditioning system.

A PV system consists of a number of

interconnected components designed to accomplish

a desired task, which may be to feed electricity into

the main distribution grid, to pump water from a

well, to power a small calculator or one of many

more possible uses of solar-generated electricity.

The design of the system depends on the task it

must perform and the location and other site

conditions under which it must operate. This

section will consider the components of a PV

system, variations in design according to the

purpose of the system, system sizing and aspects of

system operation and maintenance.

III. SYSTEM DESIGN

There are two main system configurations – stand-

alone and grid-connected. As its name implies, the

stand-alone PV system operates independently of

any other power supply and it usually supplies

electricity to a dedicated load or loads. It may

include a storage facility (e.g. battery bank) to

allow electricity to be provided during the night or

at times of poor sunlight levels. Stand-alone

systems are also often referred to as autonomous

systems since their operation is independent of

other power sources. By contrast, the grid-

connected PV system operates in parallel with the

conventional electricity distribution system. It can

be used to feed electricity into the grid distribution

system or to power loads which can also be fed

from the grid.

It is also possible to add one or more alternative

power supplies (e.g. diesel generator, wind turbine)

to the system to meet some of the load

requirements. These systems are then known as

‘hybrid’ systems. Hybrid systems can be used in

both stand-alone and grid-connected applications

but are more common in the former because,

provided the power supplies have been chosen to

be complementary, they allow reduction of the loss

of load probability. Figures below illustrate the

schematic diagrams of the three main system types.

Fig. 2.5.Schematic diagram of a stand-alone

photovoltaic system.

Fig 2.6Schematic diagram of grid-connected

photovoltaic system.

Fig2.7Schematic diagram of hybrid system

incorporating a photovoltaic array and a motor

generator (e.g. diesel or wind).

IV. PHOTOVOLTAIC INVERTER

The inverter is the heart of the PV system and is the

focus of all utility-interconnection codes and

standards. A Solar inverter or PV inverter is a type

of electrical inverter that is made to change

the direct current (DC) electricity from

a photovoltaic array into alternating current (AC)

for use with home appliances and possibly a utility

grid. Since the PV array is a dc source, an inverter

is required to convert the dc power to normal ac

power that is used in our homes and offices.

To save energy they run only when the sun is up

and should be located in cool locations away from

direct sunlight. The PCU is a general term for all

the equipment involved including the inverter and

the interface with the PV (and battery system if

used) and the utility grid. It is very important to

point out that inverters are by design much safer

than rotating generators. Of particular concern to

utility engineers is how much current a generator

can deliver during a fault on their system. Inverters

generally produce less than 20% of the fault current

as a synchronous generator of the same nameplate

capacity. This is a very significant difference.

http://en.wikipedia.org/wiki/Electrical_inverter

http://en.wikipedia.org/wiki/Direct_current

http://en.wikipedia.org/wiki/Photovoltaic_array

http://en.wikipedia.org/wiki/Alternating_current

http://en.wikipedia.org/wiki/Utility_grid

http://en.wikipedia.org/wiki/Utility_grid




V. DIODE-CLAMPED CONVERTER:

The simplest diode-clamped converter is

commonly known as the neutral point clamped

converter (NPC) which was introduced by Nabae.

The NPC consists of two pairs of series switches

(upper and lower) in parallel with two series

capacitors where the anode of the upper diode is

connected to the midpoint (neutral) of the

capacitors and its cathode to the midpoint of the

upper pair of switches; the cathode of the lower

diode is connected to the midpoint of the capacitors

and divides the main DC voltage into smaller

voltages, which is shown in Figure 3.1. In this

example, the main DC voltage is divided into two.

If the point O is taken as the ground reference, the

three possible phase voltage outputs are -1/2Vdc, 0,

or 1/2Vdc . The line-line voltages of two legs with

the capacitors are: Vdc , 1/2Vdc , 0, -1/2Vdc or –Vdc .

To generate a three-phase voltage, three phases are

necessary.

The five-level output voltage can be generated by

controlling the switches. Table 3.1 shows the

proper switching states. The switches (Sa1 and Sa’1)

and (Sa2 and Sa’2) are complementary pairs. When

Sa1 is on (Sa1 = 1), Sa’1 is off (Sa’1 = 0).

Some disadvantages of the diode-clamped

multilevel converter may be observed. Using extra

diodes in series becomes impractical when the

number of levels m increases, requiring (m-1)(m-2)

diodes per phase if all the diodes have equal

blocking voltages.

Note that the voltages for diodes in different

positions are not balanced. For example, diode Da2

must block two capacitor voltages, Da(m-2) must

block (m-2) capacitor voltages. Also, the switch

duty cycle is different for some of the switches

requiring different current ratings.

In addition, the capacitors do not share the same

discharge or charge current resulting in a voltage

imbalance of the series capacitors. The capacitor

voltage imbalance can be controlled by using a

back-to-back topology, connecting resistors in

parallel with capacitors, or using redundant voltage

states.

The advantages for the diode-clamped converter

are the following:

(1) A large number of levels yield a small harmonic

distortion.

(2) All phases share the same DC bus.

(3) Reactive power flow can be controlled.

(4) Control is simple.

Fig 3.1 Neutral Point Diode Clamped Converter

Fig 3.2 Two-Phase Diode Clamped Multi Level

Converter

Sa1 Sa2 Sa’1 Sa’2 Sb1 Sb2 Sb’1 Sb’2 Vao Vbo Vab

0 0 1 1 1 1 0 0

-

1/2Vdc 1/2Vdc -Vdc

0 0 1 1 0 1 1 0

-

1/2Vdc 0

-

1/2Vdc

1 1 0 0 1 1 0 0 1/2Vdc 1/2Vdc 0

0 0 1 1 0 0 1 1

-

1/2Vdc -1/2Vdc 0

0 1 1 0 0 0 1 1 0 -1/2Vdc 1/2Vdc

1 1 0 0 0 0 1 1 1/2Vdc -1/2Vdc Vdc




VI. SEVEN LEVEL INVERTER

INTRODUCTION

The seven-level inverter is composed of a capacitor

selection circuit and a full-bridge power converter,

which are connected in cascade. The operation of

the seven level inverter can be divided into the

positive half cycle and the negative half cycle of

the utility. For ease of analysis, the power

electronic switches and diodes are assumed to be

ideal, while the voltages of both capacitors C1 and

C2 in the capacitor selection circuit are constant

and equal to Vdc/3 and 2Vdc/3, respectively. Since

the output current of the solar power generation

system will be controlled to be sinusoidal and in

phase with the utility voltage, the output current of

the seven-level inverter is also positive in the

positive half cycle of the utility. The operation of

the seven-level inverter in the positive half cycle of

the utility can be further divided into four modes,

as shown in Fig.4.1

Mode1: The operation of mode 1 is shown in Fig.

4.1(a). Both SS1 and SS2 of the capacitor selection

circuit are OFF, so C1 is discharged through D1

and the output voltage of the capacitor selection

circuit is Vdc/3. S1 and S4 of the full-bridge power

converter are ON. At this point, the output voltage

of the seven-level inverter is directly equal to the

output voltage of the capacitor selection circuit,

which means the output voltage of the seven-level

inverter is Vdc/3.

Mode 2: The operation of mode 2 is shown in Fig.

4.1 (b). In the capacitor selection circuit, SS1 is

OFF and SS2 is ON, so C2 is discharged through

SS2 and D2 and the output voltage of the capacitor

selection circuit is 2Vdc/3. S1 and S4 of the full-

bridge power converter are ON. At this point, the

output voltage of the seven-level inverter is 2Vdc/3.

MODE 3: The operation of mode 3 is shown in Fig.

4.1(c). In the capacitor selection circuit, SS1 is ON.

Since D2 has a reverse bias when SS1 is ON, the

state of SS2 cannot affect the current flow.

Therefore, SS2 may be ON or OFF, to avoiding

switching of SS2. Both C1 and C2 are discharged

in series and the output voltage of the capacitor

selection circuit is Vdc. S1 and S4 of the full-bridge

power converter are ON. At this point, the output

voltage of the seven-level inverter is Vdc.

Fig.4.1. Operation of the seven-level inverter in

the positive half cycle, (a) Mode 1, (b) Mode 2, (c)

Mode 3, and (d) Mode 4.

VII. CONTROL MECHANISUM

The proposed solar power generation system

consists of a dc– dc power converter and a seven-

level inverter. The seven-level inverter converts the

dc power into high quality ac power and feeds it

into the utility and regulates the voltages of

capacitorsC1 and C2 . The dc–dc power converter

supplies two independent voltage sources with

multiple relationships and performs maximum

power point tracking (MPPT) in order to extract the

maximum output power from the solar cell array.

Fig.5.1. Control block: (a) seven-level inverter

and (b) dc–dc power converter.

VIII. RESULTS

TEST RESULTS

To verify the performance of the proposed solar

power generation system, a prototype was

developed with a controller based on the DSP chip

TMS320F28035. The power rating of the prototype

is 500 W, and the prototype was used for a single-

phase utility with 110Vand 60 Hz. Table 6.1 shows

the main parameters of the prototype. Figs. 6.1 and

6.2 show the experimental results for the seven

level inverter when the output power of solar

power generation system is 500 W. Fig. 6.1 shows

the experimental results for the AC side of the

seven-level inverter. Fig. 6.1(b) shows that the

output voltage of the seven-level inverter has seven

voltage levels. The output current of the seven-

level inverter, shown in Fig. 6.1 (c), is sinusoidal

and in phase with the utility voltage, which means




that the grid-connected power conversion interface

feeds a pure real power to the utility.

Table 6.1 parameters of the prototype

IX. SIMULATION AND RESULTS

Fig.6.7 simulation diagram

Fig 6.8: output waveform

Fig 6.9: modified output waveform

Fig 6.10: output after using filter

X. CONCLUSION

This project presents a solar power generation

system to convert the dc energy generated by a

solar cell array into ac energy that is fed into the

utility. The proposed solar power generation

system is composed of a dc–dc power converter

and a seven level inverter. The seven-level inverter

contains only six power electronic switches, which

simplifies the circuit configuration. Furthermore,

only one power electronic switch is switched at

high frequency at any time to generate the seven-

level output voltage. This reduces the switching

power loss and improves the power efficiency. The

voltages of the two dc capacitors in the proposed

seven-level inverter are balanced automatically, so

the control circuit is simplified. Experimental

results show that the proposed solar power

generation system generates a seven-level output

voltage and outputs a sinusoidal current that is in

phase with the utility voltage, yielding a power

factor of unity. In addition, the proposed solar

power generation system can effectively trace the

maximum power of solar cell array.

XI. REFERENCES

[1] R. A. Mastromauro, M. Liserre, and A.

Dell’Aquila, “Control issues in single-stage

photovoltaic systems: MPPT, current and

voltage control,” IEEE Trans. Ind.

Informat., vol. 8, no. 2, pp. 241–254, May.

2012.

[2] Z. Zhao, M. Xu,Q. Chen, J. S. Jason Lai,

andY. H. Cho, “Derivation, analysis, and

implementation of a boost–buck converter-

based high-efficiency pv inverter,” IEEE

Trans. Power Electron., vol. 27, no. 3, pp.

1304–1313, Mar. 2012.

[3] M. Hanif, M. Basu, and K. Gaughan,

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design example,” IET Power Electron., vol.

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[4] J.-M. Shen, H. L. Jou, and J. C. Wu, “Novel

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[5] N. Mohan, T. M. Undeland, and W. P.

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AUTHOR’s PROFILE

P.Vidhya Sri received the

B.Tech Degree from SVU in

2011, Currently pursuing her

post graduation (M.tech.) from

Gokula Krishna College of

Engineering, Sullurpet, SPSR

Nellore (Dist), A.P, India.

K. Swapna Received the

B.Tech degree in Electrical &

Electronics engineering from

Sri venkateshwara University,

Tirupati, Andhra Pradesh, India

,in 2012, the M.Tech. degree in

Electrical Engineering from

Jawaharlal Nehru Technological University

Ananthpur, spsr Nellore, Andhra Pradesh, India, in

2015. Currently, she is working as a Assistant

Professor in the Department of Electrical &

Electronics Engineering at Gokula Krishna college

of engineering, Sullurpet, SPSR Nellore District,

Andhra Pradesh, India. She has one year teaching

Experience. Her research interest includes power

system operation and control, Hvdc Transmission

systems and renewable energy sources.

P. Vidhya Sri* et al. (IJITR) INTERNATIONAL JOURNAL OF ...

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